Variable reflectance mirror

ABSTRACT

The present invention provides variable reflectance mirrors wherein the component of variable transmittance is a single-compartment, self-erasing, solution-phase electrochromic device.

This application is a continuation of application Ser. No. 760,877,filed Sep. 16, 1991, abandoned, which is a division of application Ser.No. 422,601, filed Oct. 17, 1989, abandoned, which is a division ofapplication Ser. No. 846,354, filed Mar. 31, 1986, now U.S. Pat. No.4,902,108.

TECHNICAL FIELD

The present invention relates to devices of reversibly variabletransmittance to electromagnetic radiation, compositions for use asmedia of reversibly variable transmittance in such devices, and use ofsuch devices in variable transmission light filters and variablereflectance mirrors. More particularly, the invention relates tosingle-compartment, self-erasing, solution-phase electrochromic devices,solutions for use therein and uses thereof.

BACKGROUND OF THE INVENTION

Several different types of devices are known wherein transmittance toelectromagnetic radiation can be reversibly varied. Among such devicesare those wherein the transmittance is changed by thermochromic,photochromic, or electro-optic (e.g., liquid crystal, dipolarsuspension, electrophoretic, electrochromic) means and wherein thevariable transmittance is to electromagnetic radiation that is at leastpartly in the visible range (wavelength from 4200Å to 7000Å).

Devices of reversibly variable transmittance to electromagneticradiation have found application as the variable transmittance elementin variable transmittance light-filters, variable reflectance mirrors,and display devices which employ such light-filters or mirrors inconveying information. These variable transmittance light filters haveincluded windows. The variable reflectance mirrors have includedanti-glare rearview mirrors for automotive vehicles.

Devices of reversibly variable transmittance to electromagneticradiation, wherein the transmittance is altered by electrochromic means,including electrochemichromic devices, are described, for example, byChang, "Electrochromic and Electrochemichromic Materials and Phenomena,"in Non-emissive Electrooptic Displays, A. Kmetz and K. von Willisen,eds. Pergamon Press, New York, N.Y. 1976, pp. 155-196 (1976).Electrochemichromic devices include those wherein electrochemicalreactions occur in a solid film, involve electroplating or occurentirely in solution. See Chang, supra.

Numerous electrochemichromic devices are known in the art. See, e.g.,Manos, U.S. Pat. No. 3,451,741; Bredfeldt et al., U.S. Pat. No.4,090,782; Shattuck and Sincerbox, U.S. Pat. No. 4,093,358; Clecak etal., U.S. Pat. No. 4,139,276; Kissa et al., U.S. Pat. No. 3,453,038;Rogers, U.S. Pat. Nos. 3,652,149, 3,774,988 and 3,873,185; and Jones etal., U.S. Pat. Nos. 3,282,157, 3,282,158, 3,282,160 and 3,283,656. Amongthese devices are single-compartment, self-erasing, solution-phaseelectrochromic devices. See, e.g., Manos, supra, which is incorporatedherein by reference; Bredfeldt et al., supra; Shattuck and Sincerbox,supra; and Clecak et al., supra.

In a single-compartment, self-erasing, solution-phase electrochromicdevice, the intensity of electromagnetic radiation is modulated bypassing through a solution held in the device in a compartment whichincludes two electrodes. The two electrodes are in contact with thesolution. Between the electrodes, there is no barrier, such as asemi-permeable membrane, which would divide the solution compartment andprevent some components in the solution from diffusing or migrating fromone electrode to the other. The solution includes a solvent and at leastone "anodic" compound (which can be neutral or charged) and at least one"cathodic" compound (which also can be neutral or charged). The "anodic"compounds are electrochemically oxidized and the "cathodic" compoundsare electrochemically reduced when a DC electrical potential differenceis impressed across the solution between the electrodes. If none of the"anodic" compounds and "cathodic" compounds to be oxidized or reduced ischarged, prior to oxidation or reduction, respectively, the solutionwill, and otherwise the solution may, include inert, current-carryingelectrolyte. The electrochemical properties of the solvent, inert,current-carrying electrolyte, if any, anodic compounds, cathodiccompounds, and any other components that might be present in thesolution are preferably such that the anodic and cathodic compounds areoxidized and reduced, respectively, at a potential difference betweenthe electrodes which does not cause any significant electrochemical orother changes in the other components in the solution. The solution isfluid during operation of the device, although it may be gelled or madehighly viscous with a thickening agent. That the devices are"solution-phase" means that all of the components in the solution,including the anodic and cathodic compounds, remain in solution duringoperation of the device with the concomitant oxidation of anodiccompounds and reduction of cathodic compounds.

Reversible modulation of intensity of electromagnetic radiation passingthrough a single-compartment, self-erasing, solution-phaseelectrochromic device can be accomplished because of three factorsrelated to operation of the device. First, the molar extinctioncoefficients of the anodic compounds and cathodic compounds in thesolution of the device, as a function of wavelength, change with theirelectrochemical oxidation and reduction, respectively. Generally, atleast one of these compounds undergoes a significant change inextinction coefficient at wavelengths in the visible range upon theoxidation or reduction; consequently, the solution and device changecolor or change from dark to clear or clear to dark when a potentialdifference is applied across the solution between the electrodes.Second, in the solution, the oxidized anodic compounds and reducedcathodic compounds do not, to any significant extent, undergodegradative reactions unimolecularly or with other components. Third, inthe solution, the oxidized anodic compounds react substantially onlywith the reduced cathodic compounds to yield substantially only anodiccompounds and cathodic compounds in their forms and with theirproperties prior to the oxidations and reductions, respectively. Thesereactions of oxidized anodic compounds with reduced cathodic compoundsprovide the "self-erasing" feature to the device.

Heretofore, no single-compartment, self-erasing, solution-phaseelectrochromic devices have been known which have proven to be suitablefor commercial application as the component of reversibly variabletransmittance in variable transmittance light filters or variablereflectance mirrors. For such applications, the solution of variabletransmittance must be highly stable to cycling, at least severalthousands of times, from zero potential difference between theelectrodes to a potential difference between the electrodes that issufficient to cause significant change in transmittance and then back tozero again. In a typical device, the solution is held in a layer betweenplanar, parallel, spaced-apart, transparent walls, on the insidesurfaces of which (in contact with the solution) are coated thin layersof transparent, electrically conductive material which serve aselectrodes and through which passes electromagnetic radiation whoseintensity is reversibly modulated in the device. It is advantageous tohave the solution layer as thin as possible, in order to minimizedistortion of light passing through, or passing into and reflecting outof, a device, and to reduce to durations that are acceptable forcommercial applications the "response time" required for thetransmittance of a device to achieve a new steady-state value when thepotential difference between the electrodes is changed. However, fordevices with thin solution layers, anodic and cathodic electrochromiccompounds must be found that, at concentrations in the solution at whichthey remain soluble, both at zero-potential equilibrium and whenoxidized (in the case of anodic compounds) and reduced (in the case ofcathodic compounds) when a potential difference is applied between theelectrodes, give rise to sufficiently large changes in absorbancebetween their zero-potential equilibrium states and their "activated"(i.e., oxidized or reduced) states and at the same time remainsufficiently stable to cycling to provide a commercially practicabledevice. The present invention addresses the need for solutions to makecommercially practicable single-compartment, self-erasing,solution-phase electrochromic devices.

A useful feature in such devices, that has not heretofore beenavailable, is the capability to function as a gray-scale device, i.e.,to vary continuously and rapidly in transmittance to light in thevisible wavelength range as a function of the potential differenceapplied between the electrodes of the device. Such a "gray-scale" devicewould find application in a window, which would allow light of constantintensity to pass through independently of the intensity of the lightreaching the window, and an anti-glare rearview mirror in an automobile,that would reflect light of acceptable intensity to the driverregardless of the intensity of the glare-causing light incident on themirror from headlamps of automobiles approaching the vehicle frombehind. The present invention provides gray-scaling capability insingle-compartment, self-erasing, solution-phase electrochromic devices.

A problem that has not heretofore been recognized with solution-phaseelectrochromic devices is segregation, due to both migration and naturalconvection of anodic and cathodic electrochromic compounds. Particularlyin devices that are operated continuously for long periods (more thanabout 20 minutes) with the planar surface through which light enters thedevice oriented vertically to the ground, such segregation can causeannoying and troublesome separation of color and reduction in speed ofself-erasing. The present invention addresses this segregation problem.

Variable reflectance mirrors include a variable transmittance component,which is a device which has a transmittance to visible light which isreversibly varied by thermochromic, photochromic, or electro-opticmeans, and a reflection means, which is a highly reflective surface(such as a silver layer) from which light is reflected after passingthrough a medium of reversibly variable transmittance in the variabletransmittance component. After reflecting from the reflection means, thereflected light passes back through the medium of reversibly variabletransmittance. The medium of variable transmittance in such mirrors istypically held, in the variable transmittance component, between twoplanar, parallel, spaced-apart surfaces. At least one of these surfacesis transparent to light, and light reflected by the mirror enters andleaves through this transparent surface. A problem with such mirrors isthe high "residual" reflectivity, which is usually greater than 5%, ofthis transparent surface of the variable transmittance component. Forexample, in an anti-glare rearview mirror for an automobile, whereinelimination of high glare may require reduction of reflectivity observedby the driver from all surfaces to as low as about 5 to 7%, the highresidual reflectivity of the front surface of a typical mirror requiresthat the transmittance of the medium of reversibly variabletransmittance in the mirror be capable of being made as low as about 3%.Because it is difficult to achieve such low transmittance withsufficient speed in preferably thin devices of reversibly variabletransmittance, it would be advantageous to have variable reflectancemirrors wherein these problems caused by high residual reflectivity areavoided. The present invention provides such mirrors.

SUMMARY OF THE INVENTION

The present invention provides solutions for use as the medium ofreversibly variable transmittance to electromagnetic radiation,particularly light in the visible range, in single-compartment,self-erasing, solution-phase electrochromic devices.

The invention provides further such electrochromic devices, wherein asolution of the invention is the medium of reversibly variabletransmittance; variable transmission light filters and variablereflectance mirrors, wherein the variable transmittance component is asingle-compartment, self-erasing, solution-phase device according to theinvention; and display devices wherein information is displayed byoperation of variable transmission light filters or variable reflectancemirrors according to the invention.

The solutions of the invention render commercially practical the use ofsingle-compartment, self-erasing, solution-phase electrochromic devicesand variable transmission light filters, variable reflectance mirrorsand display devices employing such filters and mirrors. The solutions ofthe invention are unexpectedly highly stable to cycling of potentialdifferences between the electrodes in devices of the invention.

In devices of the invention wherein the solution layer is desirablythin, and with concentrations of anodic and cathodic compounds in thesolution that are low enough that precipitation does not occur andproblems of segregation are substantially reduced, and at potentialdifferences between the electrodes that are low enough to avoidsignificant degradation of the solution, the solutions of the inventiondarken to an unexpectedly high absorbance to visible light withunexpectedly high speed once the potential difference is applied andclear again with unexpectedly high speed once the electrodes areopen-circuited or short-circuited. Advantageously, reversal of thepolarity of the electrodes of a device of the invention is not requiredfor clearing to occur with sufficient speed for many practicalapplications. Further, devices of the invention can advantageously beoperated as gray-scale devices.

In another aspect, the present invention entails novel electrochromiccompounds and combinations of compounds for use in solutions of theinvention.

In still another aspect, the invention includes an improved variablereflectance mirror, wherein variable reflectance is provided bythermochromic, photochromic, or electro-optic means in a device ofvariable transmittance to electromagnetic radiation. In such an improvedmirror of the invention, problems due to residual reflectivity from aplanar surface through which light enters, and after reflecting from thereflecting means, leaves the mirror are avoided by displacing thisplanar surface at a slight angle to the highly reflective planar surfaceof the mirror which is its reflecting means. Thereby, a person viewingthe mirror need not see light due to residual reflectivitysimultaneously with light that is reflected from the mirror's reflectingmeans.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 displays schematically an exploded view of two planar,transparent, electrode-bearing sides, 100 and 130, of a device of theinvention together with spacer or separating means, 11, which holds theelectrode-bearing sides apart and substantially parallel in an assembleddevice of the invention and the inside edges, 11A, 11B, 11C and 11D ofwhich, together with the electrode layers, 10A and 13A, of theelectrode-bearing sides, defines a space, 12, which, in an assembleddevice of the invention, is occupied by a solution according to theinvention that is in contact with the electrode layers.

FIG. 2 illustrates schematically a partially assembled device, 200,according to the invention. FIG. 2 shows, by cross-hatched area 14, theportion, of planar, transparent side 100 of the device, which overlaysthe solution of reversibly variable transmittance in the device andwhich, consequently, changes color, or change from clear to dark andback, as the device is operated.

FIG. 3 illustrates schematically a view of a cross-section of apartially assembled, improved variable reflectance mirror, 300,according to the invention, wherein the reflecting means is the highlyreflective layer 18A of a prism-shaped mirror, 180, laminated to surface131 of one transparent, electrode-bearing side, 130, of a variabletransmittance device according to the invention.

FIG. 4 illustrates schematically a view of a cross-section of apartially assembled, improved variable reflectance mirror, 400,according to the invention, wherein the reflecting means is a highreflectance layer, 20, on one electrode-bearing side, 130, of a variabletransmittance device according to the invention and a transparentprism-shaped object, 22, is laminated to the surface 101 of the otherelectrode-bearing side, 100, of the variable transmittance deviceaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In one of its aspects, the present invention is a solution, for use asthe variable transmittance medium in a single-compartment, self-erasing,solution-phase electrochromic device, which comprises:

(A) a solvent;

(B) at least one cathodic electrochromic compound which, in avoltammogram done with an inert electrode in room temperature, displaysat least two chemically reversible reduction waves, with the first ofsaid reductions accompanied by an increase in molar extinctioncoefficient at at least one wavelength in the visible range;

(C) at least one anodic electrochromic compound which, in a voltammogramdone with an inert electrode in the solvent at room temperature,displays at least two chemically reversible oxidation waves, with thefirst of said oxidations accompanied by an increase in molar extinctioncoefficient at at least one wavelength in the visible range; and

(D) if all cathodic and anodic compounds in their zero-potentialequilibrium states in the solution are not ionic, an inertcurrent-carrying electrolyte.

The solutions of the invention are optionally gelled or thickened bybeing combined with an agent, such as acrylic sheet material thickener,derived from, for example, the acrylic sheet material sold under thetrademark LUCITE L®.

In another of its aspects, the instant invention is asingle-compartment, self-erasing, solution-phase electrochromic devicewhich comprises, as the medium of reversibly variable transmittance tolight, a solution of the invention. The solution of reversibly variabletransmittance in a device of the invention is optionally gelled orthickened.

In another aspect, the present invention entails a variabletransmittance light-filter which comprises, as the variabletransmittance element, a single-compartment, self-erasing,solution-phase device of the invention.

In a further aspect, the invention entails a variable reflectance mirrorwhich comprises, as the variable transmittance element, asingle-compartment, self-erasing, solution-phase device of theinvention.

In a still further aspect, the invention includes a display device whichcomprises, as an information-conveying element, a variable transmittancelight filter or variable reflectance mirror according to the invention.

In another aspect, the invention includes a compound of Formula LII##STR1## wherein R₇₆ is oxygen or sulfur, R₈₀ is hydrogen ordialkylamino, wherein the alkyl groups are the same or different and areeach of 1 to 6 carbon atoms, and R₇₇, R₇₈ and R₇₉ are the same ordifferent and are each selected from hydrogen, alkyl of 1 to 6 carbonatoms, phenyl optionally substituted at any one position with an alkylgroup of 1 to 6 carbon atoms, and benzyl, optionally substituted at anyone position of the phenyl group with an alkyl group of 1 to 6 carbonatoms.

In still another aspect, the present invention includes a variablereflectance mirror which comprises a device of reversibly variabletransmittance, a planar front surface, and a planar reflecting means,

(A) said device comprising

(i) a medium of transmittance which is reversibly varied bythermochromic, photochromic, or electro-optic means, and

(ii) two planar, parallel, spaced-apart surfaces, between which themedium of reversibly variable transmittance is held and through whichlight passes prior to and after reflecting from said reflecting means;with

(B) the angle between the plane of said front surface and the plane ofsaid reflecting means being about 1° to about 5°.

In the mirrors, a significant improvement arises from the positioning ofthe plane of the reflecting means at a slight angle to the plane of thefront surface of the mirror, i.e., the surface through which lightreflected by the mirror from the reflecting means enters and leaves themirror. This positioning of these planes permits the mirror to beoriented so that light from outside the mirror that is reflected fromthe front surface of the mirror without entering the mirror (i.e., lightfrom residual reflectivity of the front surface) is not seen by theperson using the mirror while light reflected from the reflecting meansis seen by such person. Thus, to reduce reflection (including reflectedglare) from such a mirror, the residual reflectivity of the frontsurface of the mirror does not need to be overcome and, consequently,the extent to which the medium of reversibly variable transmittanceneeds to be darkened is reduced in comparison with the darkening thatwould be required if the reflecting means were parallel to the frontsurface. Further, various distortions in reflected images that occur,when both light reflected due to residual reflectivity of the frontsurface of a mirror and light reflected from the reflecting means of themirror are observed, are avoided when only light from the reflectingmeans is seen.

Although any medium whose transmittance to visible light can bereversibly altered by thermochromic, photochromic, or electro-opticmeans can be employed as the medium of reversibly variable transmittancein these improved mirrors of the invention, it is most preferred thatthe medium be a solution according to the present invention (optionallygelled or thickened) and that the device of reversibly variabletransmittance be a single-compartment, self-erasing, solution-phasedevice according to the present invention, which has two planar,parallel, spaced-apart sides, at least one of which is transparent (andthrough which light reflected by the mirror from the reflecting meanspasses prior to and after reflecting from the reflecting means) and theother of which, if not transparent, has a highly reflecting layer, whichserves as the reflecting means of the mirror, adhered to its sideopposite the side in contact with the solution.

Construction and operation of single-compartment, self-erasing,solution-phase electrochromic devices, essentially the same as those ofthe present invention but having different solutions of reversiblyvariable transmittance, are known in the art. See Manos, Bredfeldt etal., Shattuck and Sincerbox, and Clecak et al., supra.

In FIG. 1, the basic structural elements of a typical device of theinvention are illustrated in an exploded view. These elements includetwo planar electrode-bearing sides or walls, 100 and 130, a spacing orseparating layer, 11, which spaces apart and holds parallel the walls100 and 130 in an assembled device, and surrounds a space or volume, 12.Volume 12 is defined, in an assembled device, by electrode layers, 10Aand 13A, of the electrode-bearing walls 100 and 130, respectively, aswell as the four inside walls, 11A, 11B, 11C and 11D, of layer 11 (InFIG. 1, inside walls 11B and 11C are hidden from view). In an assembleddevice, volume 12 is filled (or nearly filled, in case room is left forexpansion with temperature increase) with a solution according to theinvention, which has reversibly variable transmittance in operation ofthe device. The solution in volume 12 is in contact with both electrodelayers 10A and 13A during operation of the device.

Usually, and preferably, in an assembled device, walls 100 and 130,including electrode layers 10A and 13A, and the layers 10 and 13,respectively, of the solid material to which the electrode layersadhere, are planar and parallel. By "planar" and "parallel" in thepresent specification are meant planar and parallel, respectively,within normal tolerance limits, as understood in the art, taking accountof possible slight variations arising from slight deviation inuniformity of thickness at different points (e.g., of each of layers 11,10, 10A, 13 and 13A in the Figures), flexibility of materials, and thelike.

However, it is to be understood that, as long as volume 12 can be sealedafter being filled (or nearly filled) with solution, electrode-bearinglayers 100 and 130 can be other than planar and can be spaced so thattheir inner, electrode layers are other than equidistant at each point(provided the electrode layers do not come into contact with eachother). Further, although in the preferred devices layer 100 and layer130 will be made from the same materials, having electrode layers (10A,13A) of the same thickness and same material, having solid materiallayers (10, 13) of the same thickness and same material, and otherwisebe essentially the same, it is not necessary that this be the case. Theelectrode layers, like the solid material layers, can be of differentmaterials and different thicknesses.

In typical devices of the invention, solid material layers, 10 and 13,of walls 100 and 130, respectively, will be made of clear glass or clearplastic, between 0.05 cm and 1 cm thick, which are suitable for coatingwith layers of electrically conducting material, to form electrodelayers 10A and 13A. Layers 10 and 13 can, however, be made of anymaterial which is transparent and to which an electrically conductingmaterial can be affixed to form electrode layers.

Electrode layers, 10A and 13A, can be made of any electricallyconducting material that can be adhered in a layer to the material ofsolid material layers, 10 and 13, respectively, and that is essentiallychemically inert to the solutions of reversibly variable transmittancethat are employed in devices of the invention. Suitable materials forthe electrode layers are thin, transparent layers of gold, tin oxide,ruthenium oxide, cadmium stannate, and indium-doped tin oxide ("ITO"),or thin, highly reflective layers of materials such as rhodium, orInconel. Preferred is ITO. Methods of applying the electricallyconducting material to the solid material, of layers 10 and 13, to formsuitable electrode layers are known in the art. Preferably, asillustrated in FIGS. 1 and 2, the electrode layer will cover the entiresurface of a solid material layer, over the volume 12 and spacer 11 aswell as on an extension of the solid material layer beyond an outsidewall of spacer 11 (i.e., with reference to FIG. 1, a wall of spacer 11opposite wall 11A, 11B, 11C or 11D). The electrode layer will preferablyhave a thickness that is as uniform as possible over the entire surfaceof the solid material layer to which it is affixed; the thickness of theelectrode layer will preferably be such that it has a sheet resistanceof less than 100 ohms per square and, more preferably, less than 40 ohmsper square. It is, however, not necessary that the electrode layer coverthe entire solution volume of a device of the invention or extendoutside the spacer which holds apart electrode-bearing walls of thedevice, as long as electrical contact can be made with the layer and, inoperating the device, solution in the solution space is in contact withthe electrode layer. Further, it is not required that the electrodelayer have uniform thickness or that it have a sheet resistance lessthan 100ohms per square.

It is also possible, in a device of the invention, to have one or bothelectrodes separate from solid material layers, such as 10 and 13 in theFigures. With reference to FIG. 1, in place of electrode layers 10A and13A, electrode strips could, for example, be situated along and parallelto sides 11B and 11D. Alternatively, one of electrode layers 10A and 13Acould be replaced with an electrode plate or strip parallel to but notadhered to solid material layer 10 or 13, respectively. If theelectrodes are separate from the solid material layers, the electrodesas well as the solid material layers are of material that is essentiallychemically inert to solutions of the invention. In such devices, glassis a suitable material for the solid material layers and rhodium orplatinum are suitable as electrodes.

The devices of the invention reversibly modulate the intensity of lightthat enters and leaves the device. Thus, in a device of the invention,at least part of at least one wall of the solution space is transparentto light of a range of wavelengths which includes at least a part of therange of wavelengths over which the transmittance of the solution of theinvention in the solution space is reversibly varied in operation of thedevice. In the typical device, the entire area of both walls of thesolution space will be transparent to light of all wavelengths in atleast the visible range.

In a preferred variable transmittance light filter according to theinvention, the device of reversibly variable transmittance will be adevice according to the invention wherein both walls of the solutionspace (e.g., 100 and 130 in FIG. 1) are transparent to visible light ofall wavelengths.

To prepare a variable reflectance mirror according to the invention, ahighly reflecting layer, such as of silver, can be applied to theoutside (i.e., the side opposite the solution) of one of the transparentwalls of the solution volume of a device according to the invention,wherein, but for the reflecting layer, both walls of the solution spacewould be transparent. Alternatively, a variable reflectance mirror canbe made by employing for one of the electrode layers defining thesolution space in a device, a highly reflecting, electrically conductivematerial such as rhodium or Inconel.

As described further below, transparent walls of a device of theinvention, defining the solution volume, can be joined, bonded orlaminated to plates of glass or plastic, mirrors, and the like to makevariable transmittance light filters and variable reflectance mirrorsaccording to the invention wherein variable transmittance to light isprovided by a device of the invention.

In the present specification, "transparent" to light of a range ofwavelengths means that at least some light, of all wavelengths in therange, passes through, instead of being absorbed or reflected. Use ofthe word "transparent" without qualification means transparency to lightof a range of wavelengths which includes at least all wavelengths in thevisible range (wavelength from 4200Å to 7000Å). Typically, and as apractical matter, a transparent wall of the solution volume of a deviceof the invention will allow at least about 90% of the light, at allwavelengths in the visible range, that is incident on it to passthrough, rather than be reflected or absorbed.

In contrast, a "highly reflecting" surface, within the meaning of thepresent specification, is one that reflects, rather than transmits orabsorbs, at least about 50%, and more typically at least about 70%, oflight of all wavelengths in an identified range. If used withoutqualification, a surface that is "high reflecting" is one that is so tolight of all wavelengths at least in the visible range.

The spacer, denoted as 11 in the Figures, is electrically insulating andis made of a combination of a sealing material, such as epoxy resin,silicones, rubber cement, low melting glass, certain plastics, paraffinwax, or the like, with a spacing material such as small glass beads,nylon monofilament, MYLAR® strips, polystyrene beads or the like. Asindicated above, the spacer is preferably of substantially uniformthickness so that the two walls defining the solution space in a devicecan be held essentially parallel to each other. Although shownschematically as planar in FIG. 1, the inside edges 11A, 11B, 11C and11D of the spacer, and the outside edges opposite the inside edges, arein reality curved or rough edged. This curvature or roughness will beclear from the manner by which a typical device is assembled: by placingstrips of a (highly viscous) mixture, of sealing material with spacingmaterial, around an area on the inside (i.e., electrode layer bearingside) of one wall of a device and then pressing the other wall of thedevice, with its inside (i.e., electrode-bearing side) wall facing theinside of the first wall, against the strips until both walls contactthe separating means. This pressing squeezes excess sealing material inthe strips from the strips and causes the outside and inside edges ofthe strips to be curved or uneven. In the devices of the invention, theseparating material in the spacer holds the inside (i.e.,electrode-bearing) surfaces of walls between about 0.0025 to about 0.05cm apart. A preferred spacer is a combination of glass beads with epoxyresin sealant.

The electrodes of a device of the invention are connected to, or capableof being connected to, a DC power source, whereby an electricalpotential can be impressed between the electrodes and across thesolution in a device. In the device illustrated schematically in FIG. 2,a preferred arrangement for connecting the electrodes to a power sourceis illustrated. In this arrangement, the two electrode-bearing walls aredisplaced in opposite directions, laterally from but parallel to thesolution space, in order to provide an exposed strip of each of theelectrode surfaces. To each of these exposed strips is adhered, so as tobe in electrical contact with the strip along nearly its entire exposedlength, an electrically conductive strip or wire, such as a copper,aluminum or silver strip or wire. One such strip, 16, is shown in itsentirety in FIG. 2 and in cross-section in FIGS. 3 and 4. Only the leador extension, 15A, of the other strip 15 of the device of FIG. 2 is seenin FIG. 2. Strip 15 is seen in cross-section in FIGS. 3 and 4. Likestrip 16 affixed to electrode-layer 13A, strip 15 is affixed toelectrode layer 10A along essentially the entire length of the overhangof the electrode layer. Although any means known in the art can beemployed to secure the wire or strip in electrical contact with theelectrode surface, such as clamping, soldering or securing with aconductive adhesive, a preferred means is to use a conductive epoxy,such as standard silver epoxy. The strips or wires affixed to theelectrode surfaces have leads or extensions, illustrated by 15A and 16Ain FIG. 2 beyond the ends of the electrode surfaces. Connection to asuitable power source is effected by standard electrical connection fromthe power source to these leads or extensions.

Assembly of a device of the invention can be carried out as understoodin the art. See Manos, supra. A preferred method for assembling a deviceis as follows:

A strip of spacer material, consisting of a separating material, such asglass beads, mixed with a sealing material, such as insulating epoxy, isdeposited on one surface of the device (on the electrode surfacethereof, in the preferred case wherein the surface of the device is aplanar piece of solid material, such as glass, to which is affixed oradhered a layer of electrically conducting material to serve as anelectrode) to outline a cross-sectional area, of desired size and shape,for the solution volume. The solution volume is then formed by placingthe other surface of the device over the strip of spacer material, sothat the electrode layers of the surfaces face each other, and thenapplying pressure to the two surfaces to cause them to approach eachother until they are separated substantially only by the separatingmaterial in the spacer. If the solution used with the device is to bethickened by combination with a thickener, such as acrylic sheetmaterial, as derived from LUCITE L®, a solution of the thickener in avolatile solvent such as dichloroethane, acetone or methyl ethyl Ketoneis conveniently painted or sprayed on the entire area outlined by thespacer on the first wall, and the solvent allowed to evaporate, prior toapplication of the second wall. After the assembly process, and prior tofilling with a solution, the sealing material of the spacer is allowedto cure, if necessary, to become inert to the solution; such curing isnecessary when the solvent of the solution is propylene carbonate andthe sealing material is insulating epoxy.

The shape of the solution volume, viewed in cross-section through theelectrode-bearing walls, is not constrained to be square or rectangular.It can be circular, elliptical, polygonal, in the shape of a letter ornumeral, or any desired shape.

One of the walls of a device of the invention has bored therein (priorto assembly) two small holes located, in the assembled device, over andnear the edge of, the solution volume (e.g., with reference to FIG. 1,one near inside wall 11A and the other near inside wall 11B). The deviceis filled with solution of the invention through these holes by passingsolution in through one of them while allowing air to escape out theother. After the filling, the two holes are sealed first with aconventional thermoplastic material inert to the solution andsecondarily with a sealant such as, for example, insulating epoxy.

Then conducting wires or strips, usually copper strips, are adhered,usually with a conducting epoxy such as a standard silver epoxy, to theexposed portions of both electrode surfaces. Finally, employing thesealing material used in the spacer, the wires or strips, except for theleads or projections thereof through which contact with a power sourceis made, are sealed over, as is the entire periphery of the device,i.e., the outside of the rim or sides which include the spacer.

For solvent in a solution of the invention, any compound, or mixture ofcompounds, can be employed, which is liquid over the range oftemperatures, at which the solution of the invention is to be used asthe medium of reversibly variable transmittance in a device of theinvention, and which is known to be useful as a solvent in theelectrochemical arts. As a practical matter, for convenience inpreparing the solutions and because devices of the invention usuallywill be operated over a range of temperatures which includes roomtemperature, a solvent will be liquid over at least the range between20° C. and 27° C. (i.e., room temperature). Further, it is preferred,for the sake of stability of devices of the invention, that the solventof solution of the invention not undergo electrolysis or be involved inother, irreversible chemical reactions, during storage or normaloperation of a device. Suitable as solvents are water, methanol,ethanol, acetonitrile, N,N-dimethylformamide, dimethylsulfoxide,acetone, methyl ethyl ketone, cyclopentanone, and cyclic esters,including propylene carbonate, ethylene carbonate, β-propiolactone,β-butyrolactone, gamma-butyrolactone, gamma-valerolactone,delta-valerolactone or homogeneous (i.e., single-phase) mixtures ofthem. It is preferred that the solvents be substantially free ofdissolved oxygen and, but for water, be anhydrous. Preferred solventsare the cyclic esters or combinations thereof. Most preferred ispropylene carbonate.

In a solution of the invention, there is at least one cathodicelectrochromic compound, at a concentration at 25° C. of at least 10⁻⁴ Mup to its solubility, but more usually between about 0.01M and 0.1M,which, in the solvent of the solution, as determined by standardvoltammographic techniques at an inert electrode at room temperature,has at least two chemically reversible (i.e., not necessarilykinetically reversible, as understood in the electrochemical arts)reduction waves, the first of these reductions being accompanied by anincrease in the extinction coefficient of the cathodic compound at atleast one wavelength in the visible range.

Further, in a solution of the invention, there is at least one anodicelectrochromic compound, at a concentration at 25° C. of at least 10⁻⁴ Mup to its solubility, but more usually between about 0.01M and 0.1M,which, in the solvent of the solution, as determined by standardvoltammographic techniques at an inert electrode at room temperature,has at least two chemically reversible (as understood in theelectrochemical arts) oxidation waves, the first of these oxidationsbeing accompanied by an increase in the extinction coefficient of theanodic compound at at least one wavelength in the visible range.

Usually it is intended that, upon application of a potential differenceacross the solution between the electrodes of a device of the invention,the solution change from clear to dark or change color. Thus, it isdesirable that the first chemically reversible reduction of a cathodicelectrochromic compound or first chemically reversible oxidation of ananodic electrochromic compound employed in a solution of the inventionbe accompanied by an increase in extinction coefficient, in the solventof the solution at room temperature, by a factor of at least about 10²to at least about 10³ cm⁻¹ M⁻¹ at at least one wavelength in the visiblerange.

Among the cathodic electrochromic compounds suitable for solutions ofthe invention are the known compounds of Formula II (viologens) ##STR2##wherein R₂₁ and R₂₂ are the same or different and are each selected fromalkyl of 1 to 10 carbon atoms, phenyl optionally substituted at any oneposition with chloride, bromide, iodide, cyano, or an alkyl group of 1to 4 carbon atoms, and benzyl, wherein the phenyl group is optionallysubstituted at any one position with chloride, bromide, iodide, cyano,or an alkyl group of 1 to 4 carbon atoms; and wherein X₂₃ ⁻ and X₂₄ ⁻are and are the same or different and are each selected from chloride,bromide, iodide, BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, ClO₄ ⁻ and NO₃ ⁻ ; and the knowncompounds of Formula ##STR3## wherein R₂₁ and R₂₂ are the same ordifferent and are defined as above for the compound of Formula II, R₃₁is alkylene of 1 to 10 carbon atoms, and X₃₁ ⁻, X₃₂ ⁻, X₃₃ ⁻ and X₃₄ ⁻are the same or different and each selected from chloride, bromide BF₄⁻, PF₆ ⁻, AsF₆ ⁻, ClO₄ ⁻ and NO₃ ⁻.

The preferred compounds of Formulas II and III are those wherein all ofthe anions are the same and are ClO₄ ⁻ or BF₄ ⁻. Most preferred is BF₄⁻. The preferred cations of compounds of Formula II are those whereinR₂₁ and R₂₂ are the same and are benzyl, phenyl or n-heptyl; mostpreferred is benzyl. The most preferred cation of compounds of FormulaIII is that wherein R₃₁ is --(CH₂)₄ --and R₂₁ and R₂₂ are the same andare benzyl (i.e., tetramethylenebis[4(1-benzyl-pyridine-4"-yl)pyridinium].

Among the anodic electrochromic compounds suitable for solutions of theinvention are the known compounds of Formula IV ##STR4## wherein R₄₁,R₄₂, R₄₃ and R₄₄ are the same or different and are each selected fromhydrogen, alkyl of 1 to 10 carbon atoms, phenyl optionally substitutedat any one position with chloride, bromide, iodide, cyano, or an alkylgroup of 1 to 4 carbon atoms, and benzyl, wherein the phenyl moiety isoptionally substituted at any one position with chloride, bromide,iodide, cyano, or an alkyl group of 1 to 4 carbon atoms;

the known compounds of Formula V ##STR5## R₅₁ and R₅₄ are the same ordifferent and are each selected from hydrogen and dialkylamino, whereinthe alkyl groups are the same or different and are each of 1 to 6 carbonatoms; R₅₂ is oxygen, sulfur or NR₅₅, wherein R₅₅ is the same as ordifferent from R₅₃ and both R₅₅ and R₅₃ are selected from hydrogen,alkyl of 1 to 10 carbon atoms, phenyl optionally substituted at any oneposition with chloride, bromide, iodide, cyano, or alkyl of 1 to 4carbon atoms, or benzyl, optionally substituted at any one position ofthe phenyl group with chloride, bromide, iodide, cyano, or alkyl of 1 to4 carbon atoms;

the known compounds of Formula VI ##STR6## wherein R₆₁, R₆₂, R₆₃ and R₆₄are the same or different and are each selected from alkyl of 1 to 10carbon atoms or phenyl; and R₆₅ and R₆₆ are the same or different andare each selected from hydrogen or alkyl of 1 to 10 carbon atoms,provided that both R₆₅ and R₆₆ are hydrogen or both are alkyl, and ifR₆₅ and R₆₆ are both hydrogen, not more than one of R₆₁ and R₆₂ ishydrogen and not more than one of R₆₃ and R₆₄ is hydrogen;

the known compound of Formula VIII (tetrathiafulvalene) ##STR7##

Also suitable as an anodic compound in solutions of the invention is anovel compound of the invention, of Formula VII ##STR8## wherein R₇₁ isoxygen or sulfur, R₇₅ is hydrogen or dialkylamino, wherein the alkylgroups are the same or different and are each selected from alkyl of 1to 6 carbon atoms, and R₇₂, R₇₃ and R₇₄ are the same or different andare each selected from hydrogen, alkyl of 1 to 6 carbon atoms, phenyl,optionally substituted at any one position with an alkyl group of 1 to 6carbon atoms, and benzyl, optionally substituted at any one position ofthe phenyl group with an alkyl group of 1 to 6 carbon atoms.

Most preferred among the compounds of Formula VII is that wherein R₇₁ isoxygen, R₇₅ is hydrogen and R₇₂, R₇₃ and E₇₄ and are all methyl.

Preferred among the anodic electrochromic compounds for solutions of theinvention are those of Formulas IV and V. More preferred are those ofFormula IV wherein R₄₁, R₄₂, R₄₃ and R₄₄ are the same and are methyl orphenyl, and those of Formula V wherein R₅₁ and R₅₄ are hydrogen, R₅₂ isthe same as N-R₅₃ and R₅₃ is methyl or phenyl. Most preferred areN,N,N',N'-tetramethyl-1,4-phenylene diamine and5,10-dihydro-5,10-dimethylphenazine.

Preparation of the novel compounds of the invention, of Formula VII,follows known procedures of Gilman and Dietrick (J. Amer. Chem. Soc. 79,6178 (1957)), beginning with the known compound of Formula XX ##STR9##wherein R₇₁, R₇₂ and R₇₅ are as defined above for compounds of FormulaVII, to form the potassium adduct of Formula XXI ##STR10## and thenreacting the adduct with a mixture of compounds of Formula R₇₃ I and R₇₄I where R₇₃ and R₇₄ are as defined above for the compound of Formula VIIand can be the same, to yield the desired product after crystallization.This synthetic procedure is illustrated in Example XI, with thesynthesis of the preferred N,N',N"-trimethyltriphenazinoxazine.

A solution of the invention will include inert, current-carryingelectrolyte, if none of the cathodic electrochromic compounds and anodicelectrochromic compounds, in their zero-potential equilibrium states inthe solution, is ionic, and otherwise may optionally include such inert,current-carrying electrolyte. The inert, current-carrying electrolytewill, during normal operation of a device of the invention, carrycurrent across the solution between the electrodes and, during storageor normal operation of a device, will not undergo electrolysis or otherirreversible chemical reactions with other substances in the device soas to impair the stability of the device.

The inert, current-carrying electrolyte in a solution of the inventionwill consist of any combination of substances known in the art to besuitable for inert, current-carrying electrolyte (sometimes referred toin the art as "supporting electrolyte"). Such substances include alkalimetal salts, tetraalkylammonium salts, and aluminum chloride andbromide. Preferred as cations in inert, current-carrying electrolyte insolutions of the invention are lithium, sodium, and tetraalkylammonium,wherein the alkyl groups are the same; most preferred istetra-n-butylammonium. Preferred as anions in inert, current-carryingelectrolytes in solutions of the invention are chloride, BF₄ ⁻ and ClO₄⁻ ; most preferred in BF₄ ⁻. The concentration of inert,current-carrying electrolyte, if present in the solution of theinvention, will be between 0.005M to 2M at 25° C. More preferably, itwill be between 0.05M and 0.5M at 25° C.

The solutions of the invention are for use as the variable transmittancemedium in a single-compartment, self-erasing, solution-phaseelectrochromic device. Because the devices are "solution-phase", theconcentrations of substances in the solution, for a device to beoperated over a given temperature range with the potential appliedacross the solution not exceeding a given maximum, must be such thatprecipitation of substances from the solution does not occur, both atzero-potential equilibrium and during operation of a device, whencathodic electrochromic material(s) is (are) being reduced at thecathode and anodic electrochromic material(s) is (are) being oxidized atthe anode. Generally, provided that, at zero-potential equilibrium atall temperatures in the range of intended use, all substances arepresent in the solution at concentrations below their solubilities,precipitation will not occur during operation of a device which includesthe solution as the medium of reversibly variable transmittance.

The "self-erasing" property of devices of the invention means that,after a potential difference between the electrodes of a device isdecreased or eliminated, the transmittance of the solution in the devicewill increase spontaneously, without need for reversal of the polarityof the electrodes, to a value characteristic of the new potentialdifference. The "self-erasing" feature of the devices of the presentinvention is provided by the spontaneous, apparently diffusion-limited,reactions of oxidized anodic compounds with reduced cathodic compoundsto yield anodic compounds and cathodic compounds in their respectivezero-potential equilibrium states.

It is important, in practical applications of the devices of theinvention, that both decrease in transmittance of the solution of adevice, that occurs when the potential difference between the electrodesis increased, and the increase is transmittance of the solution of adevice, that occurs with self-erasing, occur sufficiently rapidly. It isgenerally advantageous that both decrease and increase in transmittanceoccur as rapidly as possible. Until the instant invention, cathodic andanodic compounds meeting the voltammographic and colorimetric criteriaspecified above, were not combined in a solution. It has not beenrealized in the art that, by having both cathodic and the anodiccompounds in a single-compartment, solution-phase electrochromic devicethat undergo increases in absorbance in the wavelength range ofinterest, with reduction and oxidation, respectively, that the speed oftransmittance decrease could be a speed acceptable for commercialapplication of such devices without causing commercialapplication-defeating loss in the speed of transmittance increase, byself-erasing, made possible by the solution-phase characteristic of thedevices.

Further, for practical applications of devices of the invention, it isimportant that the solutions in the devices be stable, both duringperiods when the device is not being operated and during cycling (i.e.,when the potential between the electrodes of a device is cycled betweenzero or a low value to a higher value and back and, as a result, thetransmittance of the solution in the device varies reversibly betweenhigher and lower values). Lack of stability is indicated by an increasein absorbance of white light, or light of wavelengths at whichabsorbance is varied with the device, passing through the device,including the solution therein, when the solution is at zero-potentialequilibrium, i.e., equilibrium with no potential difference between theelectrodes of the device.

A problem preventing commercial application of single-compartment,self-erasing, solution-phase electrochromic devices has been the lack ofstability of the solutions of variable transmittance employed with them.While the reasons for this instability of prior art devices are notentirely clear, they might be related to the chemical instability, andhigh reactivity, with solvent and other materials, of either or both ofthe anodic and cathodic compounds, in their oxidized and reduced states,respectively, that have been used in prior art solutions. The presentinvention has solved this problem with discovery that, with cathodic andanodic electrochromic compounds satisfying the above-specifiedvoltammographic criteria, a property of the solutions of the inventionis exceedingly and unexpectedly high stability, particularly stabilityto cycling.

It has been found that the stability of the solutions of the inventionis further enhanced by minimizing in the solutions the concentration ofoxygen and, if the solvent is non-aqueous, water. Thus, optionally butpreferably, a device of the invention is flushed with dry nitrogen orother inert gas prior to being filled with solution. Standard techniquesare employed to reduce the concentrations of oxygen and, if solvent isnon-aqueous, water, in solvent and solutes used to prepare solutions andto minimize contamination of solutions with oxygen and water prior tofilling the devices with the solutions and sealing the filled devices.For example, dry nitrogen can be bubbled through solutions prior tofilling to reduce oxygen concentration. Solvent can be treated bypassing over a dessicant, such as activated alumina, to reduce watercontamination, prior to being used to prepare a solution. In addition,solutes (electrochromic compounds; inert, current-carrying electrolyte)can be dried prior to use to prepare solutions by heating to about 110°C. Alternatively, prepared solutions can be passed through a dessicant,such as activated alumina, prior to filling a device with them.

Other than any of the aforementioned measures, that might be taken toreduce the concentrations of oxygen and water in solutions of theinvention, solutions of the invention are prepared by standard methods,usually at room temperature, by simply dissolving the appropriateamounts of solutes in the solvent to achieve the desired concentrations.

Certain advantages are realized by employing thickened or gelledsolutions as the media of reversibly variable transmittance in devicesof the invention. As described supra and further below, it has beendiscovered in connection with the present invention that segregation isa problem with single-compartment, self-erasing, solution-phaseelectrochromic devices when they are operated continuously for longperiods. Gelling or thickening the solutions of the invention reducesthe significance of the segregation problem by reducing the component ofthe segregation that is due to natural convection.

Another advantage realized by using gelled or thickened solutions in thedevices of the invention relates to convenience and safety. If a deviceshould be opened, as by breaking one of the transparent sides orotherwise, a gelled or thickened solution would flow much more slowlythan a non-gelled or non-thickened one and, consequently, the ease ofcleaning up the solution would be increased and the risk of persons'contacting any noxious or harmful substances that might be present inthe solution would be reduced. In devices, wherein the transparent sidesor other elements might shatter or splinter during breakage, a gelled orthickened solution would tend to hold the broken pieces in place andthereby reduce the risk of injury that might occur if the device brokeapart.

The terms "thicken" and "gel" are used interchangeably in the instantspecification and refer to the increase in viscosity of a solution thatresults from combining it with certain substances, whether or not a truegel is formed in the process. Any substance which can thicken asolution, without reacting to form covalent bonds with solvent, inert,current-carrying electrolyte or anodic or cathodic compounds therein,can be employed to thicken or gel a solution of the invention. Thedesired amount of thickening or gelling substance can simply be combinedwith solution, just prior to filling a device, provided there issufficient time for such filling prior to the solution's becoming tooviscous. Alternatively, the desired amount of thickening or gellingsubstance can be placed into a device before or after introduction ofsolution and the mixture with solution be accomplished in situ in thesolution space of the device; an example of this method, in which thethickener is introduced before the solution, is provided in Example X.

The concentration of thickening or gelling substance employed to preparea thickened or gelled solution of the invention will vary, depending ona number of factors, as understood by the skilled. These factors includethe thickening or gelling substance employed, the solvent employed andthe desired viscosity of the thickened or gelled solution. With thepreferred solvent, propylene carbonate, and the preferred thickener forthis solvent, the composition obtained by dissolving the acrylic sheetmaterial sold under the trademark LUCITE L, in an organic solvent suchas acetone, methyl ethyl ketone or dichloroethane, the concentration ofthickener in solution will be between about 3% (w/w) and about 30%(w/w), preferably between about 5% (w/w) to about 25% (w/w), and mostpreferably between about 7% (w/w) and about 15% (w/w).

Manos, supra, lists certain other thickeners which can be employed tomake thickened or gelled solutions of the invention. It has been foundin connection with this invention, with propylene carbonate solvent,that the composition, obtained by dissolving the acrylic sheet materialsold under the trademark PLEXIGLAS in an organic solvent such asacetone, methyl ethyl ketone, or dichloroethane, can also be used forthe thickening.

The preferred thickener is obtained by mixing a solvent, such asdichloroethane (1,2-dichloroethane, 1,1-dichloroethane or mixtures ofthe 1,1 and 1,2 isomers) with the commercially available acrylic sheetmaterial, LUCITE L®, separating the resulting solution from any residue,and, finally, allowing the solvent to evaporate. The residue left afterthe solvent evaporates is the "acrylic sheet material thickener."

It has been discovered unexpectedly, in connection with the instantinvention, that using this preferred thickener is unusually convenientand exceptionally suitable for constructing devices of the inventionwhich employ propylene carbonate solutions as media of reversiblyvariable transmittance. This convenience and suitability is due to thefacts, illustrated in Example X, that a quantity of thickener can beplaced in a device by simply painting or spraying the solution of thethickener on the electrode-bearing side of a wall of the device and thenallowing the solvent to evaporate before assembling the device and thatthe thickener inside the device is spontaneously taken up by andthickens a propylene carbonate solution of the invention, after theassembled device is filled with the solution in the usual manner.

An unexpected and highly desirable property, discovered in connectionwith the instant invention, of solutions of the invention thickened withthe preferred acrylic sheet material thickener is that the time requiredfor coloring of a device wherein such a solution is employed as themedium of reversibly variable transmittance is not significantlyincreased over the time required for coloring in a device which is thesame but for having no thickener in the solution. Thus, with suchthickener, the aforementioned advantages of using a thickened solutionas the medium of reversibly variable transmittance in a device of theinvention can be realized without significant effect on the advantage,of rapid coloring, of devices which employ non-thickened solutions ofthe invention as media of reversibly variable transmittance.

To be operated, a device of the invention is connected to a power sourcecapable of establishing a potential difference of constant polaritybetween the electrodes of the device. Referring to FIGS. 1 and 2, thisconnection is effected through leads 15A and 16A of the electricallyconducting wires or strips affixed to the electrode layers of the wallsof the device so as to be in electrically conductive contact with theelectrode layers. The power source can be any AC or DC power sourceknown in the art; however, if an AC source, control elements, such asdiodes, are placed between the power source and the electrodes of thedevice to insure that the potential difference between the electrodesdoes not change in polarity with variations in polarity of the potentialfrom the source. Suitable DC power sources are storage batteries, suchas automobile batteries and dry cell batteries. The power from the powersource delivered to the electrodes of the device is controlled by anymeans known in the art so that the potential across the solution betweenthe electrodes of the device does not exceed the potential difference atwhich irreversible reactions, such as electrolysis of solvent, reductionor oxidation of inert, current-carrying electrolyte, unimoleculardegradation reactions of electrochromic compounds and the like, occur.Preferably, to make use of the gray-scaling capability of the devices ofthe invention, the control of power delivered to the electrodes of thedevice will be such that the potential can be varied, over a range fromabout 0.1 volt to a potential somewhat below that at which irreversiblereactions occur to a significant extent in the device, but held constantat any desired potential in this range. There will also be a switchingmeans associated with the power source so that the potential between theelectrodes of the device can be reduced to zero, by open-circuiting orshort-circuiting. Because, in certain instances, the additional speed inself-erasing that can be achieved by applying a potential for a briefperiod (e.g., about 0.5 to about 5 seconds) to the electrodes, withpolarity reversed from that during decreasing transmittance, the switchmeans may also include means for accomplishing such reversals. The meansfor controlling the potential delivered to the electrodes and theswitching means can be either manually or automatically operated.

In order for the electrochromic compounds in the solutions of theinvention to be oxidized and reduced, and thereby cause decrease intransmittance of the solution, the potential difference between theelectrodes must be high enough to cause a current to flow across thesolution between the electrodes. A potential difference between about0.3 volts and about 0.5 volts is usually adequate to cause current toflow and solution of the invention to begin to darken or change color.

The extent of darkening at steady state in a particular device of theinvention will depend on the potential difference between theelectrodes; because of this property the devices of the invention areuseful as "gray-scale" devices.

The maximum potential that can be applied between the electrodes of adevice without impairing the stability of the solution will, as theskilled understand, depend on a number of factors, such as the potentialat which electrolysis of solvent occurs and potentials at whichdegradative reactions of electrochromic compounds occur. Devices of theinvention wherein water is solvent in the solution will generally beoperated at less than about 1.4 volts to avoid electrolysis of water.The devices of the present invention with cyclic ester solvents can, insome cases, be operated at a potential difference as high as about 4volts across the solution layer. Generally, however, the potentialacross the solution layer in devices of the invention is kept below 2volts.

The skilled will understand that, at steady state at a given potentialacross the solution layer of a device of the invention, cathodicelectrochromic compounds are being reduced and anodic electrochromiccompounds are being oxidized continuously at the electrodes while, atthe same time and at the same rate at which electrochemical oxidationand reduction are occurring, reduced cathodic compounds are beingoxidized back, and oxidized anodic compounds reduced back, to theirzero-potential equilibrium forms by reaction of reduced cathodic withoxidized anodic compounds. The rate at which the steady-state isachieved, at a given potential across the solution of a device, isdependent on the current across the solution at the potential. Thiscurrent is generally not regarded as an independent variable inoperation of the devices, as it depends on other factors which areindependently varied, such as the conductivity of the solution in thedevice (which in turn depends on solution composition, includingcomposition of inert, current-carrying electrolyte), and the potentialacross the solution. However, the currents that flow during normaldevice operation are typically in the range of 0.1 to 20 milliamperesper square centimeter of cathode or anode area in contact with solutionlayer.

As indicated, supra, a problem that has been discovered in connectionwith the present invention is that segregation occurs insingle-compartment, self-erasing, solution-phase electrochromic devicesthat are operated continuously (i.e., held at non-zero potential) forlong periods, longer than about 20 minutes. This segregation appears tobe similar to the segregation that is encountered in operation of largescale electrochemical cells. Thus, the segregation found in devices ofthe instant invention has a component due to migration of chargedelectrochromic compounds in electrical potential gradients in thesolution layer of a device and a component due to natural convection,which arises from different local densities, one higher and one lowerthan bulk solution density, around oxidized anodic and reduced cathodicmolecules.

Segregation in devices of the invention is preferably avoided because itgives rise to annoying color separation in the solution layer of devicesof the invention and slows the rate at which the devices self-erase.

As indicated, supra, one method for reducing at least the naturalconvection component of segregation in devices of the invention is toemploy a thickened or gelled solution of the invention as the medium ofreversibly variable transmittance.

It has also been found in connection with the invention that segregationcan be substantially eliminated in a device of the invention by

(a) employing in the device a solution of the invention which (i) hasconcentrations of cathodic and anodic electrochromic compounds at thelower end of the concentration range that is acceptable for achievingsufficient reduction of transmittance in the solution for the uses inwhich the device is to be employed, and (ii) has a concentration ofcurrent-carrying electrolyte which is at least twice and preferably atleast ten times the higher of the total concentration of anodic or totalconcentration of cathodic compounds; and

(b) with reference to FIG. 2, orienting the device so that one of theconducting strips or wires (16 and the strip or wire (not shown) ofwhich lead 15A is an extension) is higher (i.e., further from thesurface of the Earth) than the other and, in applying a potential to thedevice, to decrease or maintain below the zero-potential equilibriumvalue the transmittance of the solution in the device, placing thehigher conducting strip or wire at the higher potential (so that theelectrode to which it is attached is the anode).

For example, when oriented as just described, devices of the inventionwhich have as medium of variable transmittance the solution described inExample XII, when operated continuously at 1.0 volts for 24 hours showno appreciable segregation.

In its final aspect, the instant invention relates to improved variablereflectance mirrors, preferred embodiments of which are illustratedschematically, in cross-sectional views, in mirrors 300 and 400 of FIGS.3 and 4, respectively. As described, supra, the improvement in thesemirrors arises from the positioning of the planar reflecting means,shown as 18A in FIG. 3 and 20 in FIG. 4, at a slight angle to the planarfront surface of the mirror, which is shown as surface 101 of solidmaterial layer 10 of wall 100 of mirror 300 in FIG. 3 and surface 221 ofprism-shaped piece 22 of mirror 400 in FIG. 4. The front surface of themirror is the surface through which light passes to enter and leave themirror.

These mirrors of the invention comprise a device of reversibly variabletransmittance through which light passes before and after reflectingfrom the reflecting means.

The device of reversibly variable transmittance is characterized by twoplanar, parallel, spaced-apart surfaces which are transparent to lightof at least the wavelengths at which reflectance of the mirror isvaried, and preferably to light of all wavelengths in at least thevisible range, and between which is located a medium of absorbance whichis reversibly variable by thermochromic, photochromic or electro-opticmeans in operation of the device. With reference to mirror 300illustrated in FIG. 3 and mirror 400 illustrated in FIG. 4, thesesurfaces are surface 101 of solid material layer 10 and surface 131 ofsolid material layer 13.

Although, in mirrors 300 and 400 of FIGS. 3 and 4, respectively, thedevices of reversibly variable transmittance, with surfaces 101 and 131,are electrochromic devices that are substantially the same as the deviceof the present invention illustrated in FIG. 2, the improved mirrors ofthe invention are not limited to having single-compartment,self-erasing, solution-phase electrochromic devices according to theinstant invention as the device of reversibly variable transmittance.Any device of transmittance varied by thermochromic, photochromic orelectro-optic means can be employed to vary the reflectance of animproved mirror of the invention, provided that the medium of variabletransmittance is held in such device between two planar, parallel,spaced-apart surfaces which are transparent to light of at least thewavelengths at which the reflectance of the mirror is to be varied. Anumber of types of electro-optic devices, suitable for this purpose, areknown (e.g., liquid crystal devices, dipolar suspension devices,electrophoretic devices, two-compartment electrochemichromic devicessuch as described by Kissa, supra).

In one type of improved, variable reflectance mirror according to theinvention, which is illustrated by mirror 300 of FIG. 3, a prism-shapedmirror, 180, is laminated through a transparent laminating material,indicated by layer 19, to a surface, 131, of the device of reversiblyvariable transmittance. The prism-shaped mirror could be, for example, aconventional, prism-shaped mirror employed in rearview mirrors ofautomobiles. The prism-shaped mirror consists essentially of aprism-shaped piece, 18, of transparent solid material, such as of glassor a clear plastic, and a layer, 18A, of highly reflective material,such as silver, adhered to a surface of the solid material by anystandard technique in the mirror-fabricating art so that a highfraction, preferably at least about 80%, of the light passing throughthe solid material and reaching the reflective material layer isreflected back through the solid material. The highly reflective surfaceof the prism-shaped mirror covers at least the entire cross-sectionalarea, illustrated by 14 in FIG. 2 but not shown in the cross-sectionalview of FIG. 3, of reversibly variable transmittance of the device ofreversibly variable transmittance of the improved mirror. Highlyreflective layer 18A is the reflecting means of the improved mirror ofthe invention.

In another type of improved variable reflectance mirror according to theinvention, similar to that illustrated in FIG. 3, the layer oflaminating material is not present. Instead, the surface of theprism-shaped mirror which is not coated with a highly reflective layeris coated with an electrically conducting layer, to function as anelectrode of the device of reversibly variable transmittance, and theprism-shaped mirror, with electrode layer, replaces wall 130 as one wallof said device.

In still another type of improved, variable reflectance mirror accordingto the invention, which is illustrated by mirror 400 of FIG. 4, thereflecting means is a layer, 20, of highly reflective material, such assilver, adhered, by any standard technique in the mirror-fabricatingart, to surface 131 of the device of reversibly variable transmittanceso that a high fraction, preferably at least about 70%, of the lightpassing through the device of reversibly variable transmittance thatreaches the reflective material is reflected back through surface 131.Further, in the type of improved mirror illustrated by mirror 400 ofFIG. 4, the surface, illustrated by 101, of the device of reversiblyvariable transmittance, that is parallel to and spaced-apart fromsurface 131, is laminated through a transparent laminating material,indicated by layer 21, to a prism-shaped piece, 22, of transparent solidmaterial, such as glass or clear plastic, one surface, 221, of which isthe front surface of the improved mirror through which light reflectedby reflecting means 20 enters and leaves the mirror. The highlyreflecting layer 20 and prism-shaped piece 22 cover at least the entirecross-sectional area, illustrated by 14 in FIG. 2 but not shown in thecross-sectional view of FIG. 4, of reversibly variable transmittance ofthe device of reversibly variable transmittance of the improved mirror.

In yet another type of improved variable reflectance mirror according tothe invention, similar to that illustrated in FIG. 4, the layer oflaminating material is not present and electrode-bearing wall 100 isreplaced with the prism-shaped piece of material on one surface of whichis coated a layer of electrically conducting layer to serve as anelectrode of the device of reversibly variable transmittance.

In the improved mirrors of the invention, the angle between the plane ofthe reflecting means or layer (e.g., layer 18A in FIG. 3 and layer 20 inFIG. 4) and the front surface (e.g., surface 101 in FIG. 3 and surface221 in FIG. 4) is preferably about 1° to about 5°.

The laminating material, of layer 19 of mirror 300 of FIG. 3 and layer21 of mirror 400 of FIG. 4, can be any transparent laminating materialknown in the art. Further, the process of laminating prism-shaped mirror180 to surface 131 in mirror 300 or prism-shaped solid piece 22 tosurface 101, is by any laminating process known in the art. In apreferred improved mirror of the invention, such as mirror 300,characterized by having the reflecting means be the reflecting means ofa prism-shaped mirror, surface 131 will be of a piece of glass, solidmaterial element 18 of the prism-shaped mirror will be made of glass andthe transparent laminating material will be polyvinyl butyral (PVB).Similarly, in a preferred improved mirror of the invention, such asmirror 400, characterized by having the reflecting means adhereddirectly to one surface of the device of reversibly variabletransmittance and having a prism-shaped piece of solid-materiallaminated to the surface of the device of reversibly variabletransmittance, which is parallel to and spaced-apart from the surface towhich the reflecting means is adhered, surface element 101 will be of apiece of glass, the prism-shaped piece of material will be made ofglass, and the transparent laminating material will be PVB.

In FIGS. 3 and 4, wall 100, and elements 10 and 10A thereof; wall 130,and elements 13 and 13A thereof; spacer 11; solution space 12; and wireor strip 16 correspond to the same-numbered elements of device 200illustrated in FIG. 2. Wire or strip 15 in FIGS. 3 and 4 extends to alead or extension which corresponds to lead 15A shown in FIG. 2.

A mirror of the invention is usually mounted in a frame which shieldsfrom view all of the device of reversibly variable transmittance exceptmost of the cross-sectional area (indicated by 14 in device 200 of FIG.2) of reversibly varied transmittance through which light reflected bythe reflecting means of the mirror and seen by the observer of themirror passes before and after reflecting from the reflecting means. Theorientation of the frame can be manually or automatically adjustable.The leads 15A and 16A (illustrated in FIG. 2) of the device will beconnected to power supply control elements (e.g., switching means, meansfor controlling potential difference between the electrodes), which mayoptionally be located in the frame structure behind the device and thereflecting means or can be completely separate from the frame andmounting, and which, in turn, are connected to a power supply, such as abattery. Said power supply, particularly if small batteries, can also belocated in the frame structure; usually, however, the power supply(e.g., an automobile battery) will be located outside the frame. Thepreferred application of the variable reflectance mirrors of theinvention is as anti-glare rearview mirrors for automobiles.

When employed as the variable transmittance component of a variabletransmittance light filter, particularly a window, a device of theinvention will be framed essentially like a pane of glass in an ordinarywindow or windshield. All of the device, other than the portion thereofcorresponding to most of the cross-sectional area (indicated by 14 inthe device 200 of FIG. 2), of reversibly varied transmittance, will behidden from view by window frame components. Similarly wires from leads,15A and 16A of the device (illustrated in FIG. 2) will be run insidesuch frame components, out of the view through the window, to powersupply means and power supply control elements outside the windowstructure.

Display devices can be made with either or both of variable reflectancemirrors and variable reflectance light filters of the invention, whereindevices according to the invention are the variable transmittancecomponents, and which, through variation in reflectance ortransmittance, convey information. The area of the device of theinvention that transmits or reflects light with variable intensity canbe made to have the shape of desired symbols for a display device.Alternatively, separate devices of the invention can be arranged insuitable arrays to have the shape of desired symbols. In one embodiment,as the transmittance of the device or devices is decreased, the symbolrepresented becomes apparent to the viewer, as the device forms the darksymbol on a light background. In another embodiment, if the symbol isapparent at high transmittance of the device, because the symbol issurrounded by a dark background, activation of the device or deviceswill decrease transmittance and cause the symbol to fade from view.Virtually any symbol can be displayed with a display device employing adevice of the invention as variable transmittance component, includingletters, numerals, words, numbers or various designs. Display devicesemploying the variable transmittance devices according to the inventionare also useful in artistic displays, such as stained glass windows withpanes of reversibly variable color.

The invention is illustrated in more detail in the following examples.

Unless specified otherwise, all concentrations cited in the examples areat room temperature (20°-27° C.) and all temperatures are in degreesCelsius.

EXAMPLE I

A cell was formed by two sheets of glass 7.6 cm×12.7 cm in area andseparated by 0.020 cm thick strips of Nylon monofilament. The sheets ofglass had been coated on one side with transparent conductive electrodesof indium-doped tin oxide (ITO), and these sides were placed so as toface each other on the inside of the cell. As illustrated in FIG. 2, thesheets were slightly offset from one another to provide two parallel,narrow, overhanging strips of ITO coating, along the 12.7 cm side ofeach of the sheets, on opposite sides of the volume for solution.Contacts were made by adhering, with conductive silver epoxy, copperstrips along the narrow, overhanging strips of ITO coating and then theedges of the cell were sealed with insulating epoxy. Prior to finalsealing, the space between the electrodes was filled with a propylenecarbonate solution of 0.05M N,N,N',N'-tetramethyl-1,4-phenylenediamine,0.05M 1,1'-diheptyl-4,4'-bipyridinium difluoroborate and 0.5Mtetra-n-butylammonium fluoroborate.

When 1.0 volts was applied between the electrodes, the solution, whichinitially appeared colorless, changed to a deep blue-purple color. Thesolution returned to its bleached, colorless state when the cell wasopen-circuited or when the cell was short-circuited. The cell returnedto its bleached state more rapidly when the polarity of the 1.0 voltpotential between the electrodes was reversed for several seconds andthen the cell was short-circuited.

When the surface (opposite the ITO-coated surface) of one of the glasssheets was silvered, the device, when viewed through the unsilveredglass side, became a variable reflectance mirror.

EXAMPLE II

A cell that acted as a variable reflectance mirror was formed by twosheets of glass 10.2 cm×10.2 cm in area and spaced by 0.013 cm thickbeads of glass. One side of one of the glass sheets was coated with ITOand one side of the other sheet of glass was coated with avacuum-deposited layer of Inconel metal. The cell was assembled with theITO and Inconel electrode layers facing each other on the inside of thecell. The copper-strip contacts to the electrode surfaces, sealing andconfiguration of the device were the same as for the cell in Example I.The space between the electrodes was filled with a solution of 0.02M5,10-dihydro-5,10-dimethylphenazine, 0.02M tetramethylenebis[4(1-benzylpyridine-4'-yl)pyridinium]tetrafluoroborate, and 0.1Mtetra-n-butylammonium fluoroborate in propylene carbonate.

The reflectance from the Inconel electrode rapidly decreased when apotential of 1.0 volts was applied between the ITO and Inconelelectrodes. The applied potential caused the solution layer to turn deepblue-green. Removal of the applied potential caused the solution toreturn to its clear, zero-potential equilibrium condition and thereflectance from the Inconel electrode to increase to the original highlevel, prior to application of the potential difference.

EXAMPLE III

A device that acted as a variable transmittance light-filter or windowwas fabricated by spacing two sheets of glass, coated on one side withITO, 0.013 cm apart, using glass beads for spacing. The dimensions ofthe sheets of glass were 6.4 cm×25.4 cm. the ITO-coated sides of thesheets were facing. The copper-strip contacts, sealing and configurationof the device were the same as in the device of Example I, with thestrips along the 25.4 cm sides of the sheets. The space between theelectrodes was filled with a solution of 0.05M1,1'-dibenzyl-4,4'-bipyridinium difluoroborate and 0.05M5,10-dihydro-5,10-dimethylphenazine in propylene carbonate.

Application of a potential of 1.1 volts between the electrodes, acrossthe solution layer, caused the white light transmittance of the deviceto decrease from 81.5% to 10.0% in 11 seconds. The steady-statetransmittance of the device with 1.1 volts applied was 6.0%. Thetransmittance of the device, upon short-circuiting the electrodes,increased from 10% back to 70% in a period of 7 seconds and thetransmittance increased back to 81.5% within 16 seconds after theelectrodes were shorted. The device was cycled 40,000 times at roomtemperature between its transmittance at zero-applied potential and itssteady-state transmittance with 1.1 volts applied between theelectrodes. After the 40,000 cycles, the transmittance of the device atzero-applied potential was 78.5% and the steady-state transmittance at1.1 volts applied potential remained at 6.0%. The speed of changes intransmittance was unchanged by the cycling.

When the device was cycled 20,000 additional times at 55° C., betweentransmittance at zero applied potential and steady-state transmittanceat 1.1 volts, the transmittance at zero-applied potential decreased to71.5% while that at 1.1 volts remained at 6.0%.

EXAMPLE IV

A device that acted as a variable transmittance light filter or windowwas fabricated like the device of Example III, except that the spacebetween the electrodes was filled with a solution of 0.04M1,1'-di(n-heptyl)-4,4'-bipyridinium difluoroborate, 0.04M5,10-dihydro-5,10-dimethylphenazine and 0.1M tetra-n-butylammoniumfluoroborate in propylene carbonate.

Application of a potential of 1.1 volts between the electrodes, acrossthe solution layer, caused the white light transmittance of the deviceto decrease from 84.5% to 20.0% in a period of 10 seconds. Thesteady-state transmittance of the device with 1.1 volts applied was11.0%. The transmittance of the device, upon short-circuiting theelectrodes, increased from 20% back to 70% in a period of 7 seconds andthe transmittance increased back to 84.5% within 22 seconds after theelectrodes were shorted. The device was cycled 40,000 times at roomtemperature between its transmittance at zero-applied potential and itssteady-state transmittance with 1.1 volts applied between theelectrodes. After the 40,000 cycles, the zero-applied potentialtransmittance was 84.0% and the transmittance at 1.1 volts appliedpotential was 11.0%. The speed of changes in transmittance was unchangedby the cycling.

When the device was cycled 20,000 additional times at 55° C., betweentransmittance at zero-applied potential and steady-state transmittanceat 1.1 volts, the transmittance at zero-applied potential decreased to77.5% while that at 1.1 volts remained at 11.0%.

EXAMPLE V

A device that acted as a variable transmittance light filter or windowwas fabricated like the device of Example III, except that thedimensions of the sheets of ITO-coated glass were 6.4 cm×7.6 cm. Thesolution between the electrodes was 0.05M in1,1'-dibenzyl-4,4'-bipyridinium difluoroborate and 0.05M in5,10-dihydro-5,10-dimethylphenazine in propylene carbonate.

Application of a potential of 1.1 volts between the electrodes, acrossthe solution layer, caused the white light transmittance of the deviceto decrease from 81.5% to 10.0% in a period of 10 seconds. Thesteady-state transmittance of the device with 1.1 volts applied was11.0%. The transmittance of the device, upon short-circuiting theelectrodes, increased from 20% back to 70% in a period of 6 seconds andthe transmittance increased back to 81.5% within 15 seconds after theelectrodes were shorted. The device was cycled 40,000 times at 55° C.between its transmittance at zero-applied potential and its steady-statetransmittance with 1.1 volts applied between the electrodes. After the40,000 cycles, the zero-applied potential transmittance was 65.0% andthe steady-state transmittance at 1.1 volts applied potential remainedat 6.0%. The speed of changes in transmittance was unchanged by thecycling.

EXAMPLE VI

A device that acted as a variable transmittance light filter or windowwas fabricated like the device of Example III, except that the spacebetween the electrodes was filled with a solution of 0.01MN,N,N',N'-tetramethyl-1,4-phenylenediamine, 0.01M5,10-dihydro-5,10-dimethylphenazine, 0.01M1,1'-dibenzyl-4,4'-bipyridinium difluoroborate, 0.01M tetramethylenebis[4(1-benzylpyridine-4'-yl)pyridinium]tetrafluoroborate, and 0.1Mtetra-n-butyl ammonium fluoroborate in propylene carbonate.

Application of a potential of 1.2 volts between the electrodes, acrossthe solution layer, caused the white light transmittance of the deviceto decrease from 84% to 10% in a period of four seconds. Steady-statetransmittance with 1.2 volts was 5%. Upon short circuiting of theelectrodes, the transmittance of the device increased from 10% to 70% ina period of 6.5 seconds and increased back to the zero-potentialequilibrium value of 84% within 15 seconds after the electrodes wereshorted.

EXAMPLE VII

Devices, fabricated in essentially the same way as the deviceillustrated in Example III and filled with propylene carbonate solutionsof the electrochromic compound combinations indicated below in TableVII, were found to operate as self-erasing, solution-phaseelectrochromic devices, similarly to those illustrated in Examples I toVI.

                  TABLE VII                                                       ______________________________________                                        ANODIC          CATHODIC                                                      ELECTROCHROMIC  ELECTROCHROMIC                                                COMPOUND        COMPOUND                                                      ______________________________________                                        1.  5,10-dihydro-5,10-                                                                            1,1'-di(n-heptyl)-4,4'-bipyridinium                           dimethylphenazine                                                                             difluoroborate                                            2.  5,10-dihydro-5,10-                                                                            1,1'-di(n-heptyl)-4,4'-bipyridinium                           dimethylphenazine                                                                             diperchlorate                                             3.  5,10-dihydro-5,10-                                                                            1,1'-diphenyl-4,4'-bipyridinium                               dimethylphenazine                                                                             difluoroborate                                            4.  10-methylphenothiazine                                                                        1,1'-di(n-heptyl)-4,4'-bipyridinium                                           difluoroborate                                            5.  10-ethylphenoxazine                                                                           1,1'-di(n-heptyl)-4,4'-bipyridinium                                           difluoroborate                                            6.  tetrathiafulvalene                                                                            1,1'-dibenzyl-4,4'-bipyridinium                                               difluoroborate                                            ______________________________________                                    

EXAMPLE VIII

Numerous compounds have been tested for acceptability as anodic orcathodic electrochromic compounds in the single-compartment,self-erasing, solution-phase devices of the invention, with propylenecarbonate as solvent.

Some compounds were found to be unacceptable because of instability uponreduction (cathodic compounds) or oxidation (anodic compounds). Suchinstability is indicated by the absence of any, or the presence of onlyone, chemically reversible reduction wave (in the case of a cathodiccompound) or chemically reversible oxidation wave (in the case of ananodic compound) in a voltammogram, obtained by any standard technique,of the compound in the solvent at room temperature.

No compound, which has at least two chemically reversiblevoltammographic reduction waves (if a cathodic compound) or at least twochemically reversible voltammographic oxidation waves (if an anodiccompound) in a solvent, has been found to lead to unacceptableinstability, particularly to cycling, when combined, in a solution inthe solvent, with any other compound or compounds with the sameproperty. This observation applies particularly to such combinationswhich include at least one cathodic compound and at least one anodiccompound.

Clearly, to be acceptable, a compound must, upon reduction or oxidationin the solvent, undergo a change in extinction coefficient at at leastone wavelength in the visible range (4200Å to 700Å). To insurestability, such a change must occur with the reduction corresponding tothe first, of at least two, chemically reversible voltammographicreduction waves, if the compound is a cathodic compound, or theoxidation corresponding to the first, of at least two, chemicallyreversible voltammographic oxidation waves, if the compound is an anodiccompound.

Beyond being minimally acceptable as a cathodic or anodic electrochromiccompound in a solution of the invention, a compound will desirably havea solubility, in its zero-potential equilibrium state in the solvent ofsuch a solution, of at least about 10⁻⁴ M at 25° C. and will undergo, atat least one wavelength in the visible range, upon the reductioncorresponding to the first chemically reversible voltammographicreduction wave, if a cathodic compound, or the oxidation correspondingto the first chemically reversible voltammographic oxidation wave, if ananodic compound, an increase in extinction coefficient by at least afactor of about 10² to at least about 10³ cm⁻¹ M⁻¹.

Compounds that have been found to meet these criteria of acceptabilityand desirability, with propylene carbonate as solvent, are all of thosespecifically mentioned in any of Examples I to VII, and, in addition,the novel anodic compound, N,N',N"-trimethyltriphenazinoxazine, theknown anodic compounds, o-tolidine, N,N,N',N'-tetramethylbenzidine,N,N,N',N'-tetraphenyl-1,4-phenylene diamine, and5,10-dihydro-5,10-diphenylphenazine and the known cathodic compounds1,1'-dimethyl-4,4'-bipyridinium dichloride, 1,1'-di(p-cyanophenyl)-4,4'-bipyridinium difluoroborate, and1,1'-diphenyl-4,4'-bipyridinium diiodide.

EXAMPLE IX

This example illustrates that devices of the invention are useful asgray-scale devices, i.e., devices in which, by adjusting potentialdifference between the electrodes, transmittance can be adjusted to, andstabilized at, intermediate values between the "clear" (i.e.,zero-potential equilibrium) value and the darkest value that is possibleto attain without impairing chemical stability.

A cell like that of Example III was constructed and filled with asolution which was 0.04M in 1,1'-di(n-heptyl)-4,4'-bipyridiniumdifluoroborate and 0.04M in 5,10-dihydro-5,10-dimethylphenazine inpropylene carbonate. The steady-state transmittance of the cell to whitelight was measured as a function of the potential difference between theelectrodes of the device, and the values indicated in Table IX wereobtained.

                  TABLE IX                                                        ______________________________________                                        Potential between                                                                             Steady-State                                                  the Electrodes  Transmittance                                                 (volts)         (%)                                                           ______________________________________                                        0.0             83                                                            0.1             83                                                            0.2             83                                                            0.3             83                                                            0.4             81.5                                                          0.5             71.5                                                          0.6             56.0                                                          0.7             42.0                                                          0.8             31.0                                                          0.9             24.0                                                          1.0             17.0                                                          1.1             13.0                                                          1.2             11.5                                                          ______________________________________                                    

EXAMPLE X

A device that acted as a variable reflectance mirror with a thickenedsolution was fabricated by coating the ITO surface of an ITO-coatedpiece of glass with a dichloroethane solution of the acrylic sheetmaterial sold under the trademark LUCITE L. Upon evaporation of thedichloroethane, a thin film of acrylic sheet material weighing 0.29grams was left on the ITO surface. This same piece of glass had aconventional-mirror, silvered coating on the side opposite theITO-acrylic material side and was used to prepare a cell by spacing theITO-acrylic side 0.013 cm from the ITO side of a second piece of glasswhich had only an ITO coating on one side. Spacing was with glass beads.The dimensions of the sheets of glass were 6.4 cm×25.4 cm. Thecopper-strip contacts, sealing and configuration of the device were thesame as that in Example III. The space between the ITO-acrylic side ofthe one piece of glass and the ITO side of the other piece of glass wasfilled with a solution of 0.04M 1,1'-di-n-heptyl-4,4'-bipyridiniumdifluoroborate, 0.04M 5,10-dihydro-5,10-dimethyl phenazine and 0.1Mtetrabutylammonium fluoroborate in propylene carbonate.

Within several hours at room temperature, the acrylic layer haddissolved in the propylene carbonate solution, resulting in thickening,and the device could be operated as a variable reflectance mirror byvarying the potential across the solution between the ITO electrodelayers. With an applied voltage of 1.2 volts, the reflectance changedfrom 73.5% to 20.0% in a period of 2.5 seconds and reached a steadystate reflectance of 9.0%. Upon short circuiting the electrodes, thereflectance increased from 9.0% to 60.0% in a period of 17 seconds andeventually increased back to the clear, zero-potential value of 73.5%.

EXAMPLE XI

Synthesis of N,N',N"-trimethyltriphenazinoxazine ##STR11##

The compound was made, starting with the known compound,N-methyltriphenazinoxazine, of Formula ##STR12## by following theprocedure described by Gilman and Dietrich, J. Amer. Chem. Soc. 79, 6178(1957), for converting phenazine to 5,10-dihydro-5,10-dimethylphenazine.100 milligrams of the starting compound (0.33 mmoles), 25 milligrams ofpotassium metal (0.67 mmoles) and 5 ml of ethylene glycol dimethyl etherwere stirred for 12 hours. Then an excess of methyl iodide was added,followed by absolute ethanol to destroy excess potassium.

The reaction mixture was then mixed with water. The resultingprecipitate was recrystallized from ethanol to yield approximately 2milligrams of pure product.

In propylene carbonate, the product was found to have chemicallyreversible oxidation waves and color changes very similar to those of5,10-dihydro-5,10-dimethylphenazine.

EXAMPLE XII

A device with the configuration illustrated in FIG. 3 was fabricated bylaminating, using a standard procedure with the clear laminatingmaterial polyvinylbutyral (PVB), an electrochromic device like that inExample III to a conventional, prism-shaped, automobile rearview mirror.The device was filled with a solution of 0.02M1,1'-dibenzyl-4,4'-bipyridinium difluoroborate, 0.02M5,10-dihydro-5,10-dimethylphenazine, and 0.1M tetra-n-butylammoniumfluoroborate in propylene carbonate. This device was used as therearview mirror inside an automobile. During operation, the deviceprovided a distortion-free, continuously variable reflectance (i.e.,gray-scale) mirror which was extremely effective in eliminating glaredue to headlights on vehicles approaching from behind during nightdriving.

The device was operated at zero-potential difference when there waslittle or no glare from headlights of vehicles approaching from behind,0.6 volts potential difference when there was moderate glare, and 1.0volts when there was high glare.

The clear state reflectance from the silvered surface of the prismmirror at zero applied potential was greater than 70% of the lightincident on the device. The steady state reflectance from the silveredsurface at 0.6 volts applied potential was about 30% and at 1.0 voltsapplied potential the reflectance was about 10%.

Although the invention has been described with some specificity, thoseof skill will recognize numerous variations and modifications of thespecifics that are within the spirit of the invention. The variationsand modifications are also within the scope of the invention asdisclosed and claimed herein.

What is claimed is:
 1. A variable reflectance mirror which comprises a single-compartment, self-erasing, solution-phase electrochromic device which comprises, as the medium of reversibly variable transmittance to light reflected by the mirror, a solution which comprises(A) a solvent; (B) at least one cathodic electrochromic compound which, in a voltammogram done in the solvent at room temperature, displays at least two chemically reversible reduction waves, with the first of said reductions accompanied by an increase in molar extinction coefficient at at least one wavelength in the visible range; (C) at least one anodic electrochromic compound which, in a voltammogram done in the solvent at room temperature, displays at least two chemically reversible oxidation waves, with the first of said oxidations accompanied by an increase in molar extinction coefficient at at least one wavelength in the visible range; and (D) if all cathodic and anodic compounds in their zero-potential equilibrium states in the solution are not ionic, an inert current-carrying electrolyte.
 2. A mirror according to claim 1 wherein, in the solution of reversibly variable transmittance,(A) the solvent is liquid at room temperature and selected from propylene carbonate, gamma-butyrolactorne, gamma-valerolactone, and any homogeneous mixture that is liquid at room temperature of any two or more of said solvents or any one or more of said solvents with ethylene carbonate; (B) the cathodic compounds are selected from (i) compounds of Formula II ##STR13## wherein R₂₁ and R₂₂ are the same or different and are each selected from alkyl of 1 to 10 carbon atoms, phenyl optionally substituted at any one position with chloride, bromide, iodide, cyano, or an alkyl group of 1 to 4 carbon atoms, and benzyl, wherein the phenyl group of the benzyl group is optionally substituted at any one position with chloride, bromide, iodide, cyano, or an alkyl group of 1 to 4 carbon atoms; and wherein X₂₃ and X₂₄ are the same or different and are each selected from chloride, bromide, iodide, BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, ClO₄ ⁻ and NO₃ ⁻ ; and(ii) compounds of Formula III ##STR14## wherein R₂₁ and R₂₂ are the same or different and are defined as above for the compound of Formula II, R₃₁ is alkylene of 1 to 10 carbon atoms, and X₃₁ ⁻, X₃₂ ⁻, X₃₃ ⁻, X₃₄ ⁻, are the same or different and are each selected from chloride, bromide, iodide, BF₄ ⁻, PH₆ ⁻, AsF₆ ⁻, ClO₄ ⁻ and NO₃ ⁻ ; (C) the anodic compounds are selected from(i) compounds of Formula IV ##STR15## wherein R₄₁, R₄₂, R₄₃ and R₄₄ are the same or different and are each selected from hydrogen, alkyl of 1 to 10 carbon atoms, phenyl optionally substituted at any one position with chloride, bromide, iodide, cyano, or an alkyl group of 1 to 4 carbon atoms, and benzyl, wherein the phenyl moiety of the benzyl group is optionally substituted at any one position with chloride, bromide, iodide, cyano, or an alkyl group of 1 to 4 carbon atoms. (ii) compounds of Formula V ##STR16## wherein R₅₃ are the same or different from R₅₅ and both R₅₃ and R₅₅ are selected from alkyl of 1 to 10 carbon atoms, phenyl optionally substituted at any one position with chloride, bromide, iodide, cyano, or alkyl of 1 to 4 carbon atoms, or benzyl, optionally substituted at any one position of the phenyl group with chloride, bromide, iodide, cyano, or alkyl of 1 to 4 carbon atoms; ##STR17## wherein R₆₁, R₆₂, R₆₃ and R₆₄ are the same or different and are each selected form alkyl of 1 to 10 carbon atoms or phenyl; and R₆₅ and R₆₆ are the same or different and are each selected from hydrogen or alkyl of 1 to 10 carbon atoms, provided that both R₆₅ and R₆₆ are hydrogen or both are alkyl, and if R₆₅ and R₆₆ are both are hydrogen, not more than one of R₆₁ and R₆₂ is hydrogen and not more than one of R₆₃ and R₆₄ is hydrogen; (iv) compounds of Formula VII ##STR18## wherein R₇₁ is oxygen or sulfur, R₇₅ is hydrogen or dialkylamino, wherein the alkyl groups are the same or different and are each selected from alkyl of 1 to 6 carbon atoms, and R₇₂, R₇₃. R₇₄ are the same or different and are each selected from hydrogen, alkyl of 1 to 6 carbon atoms, phenyl, optionally substituted at any one position with an alkyl group of 1 to 6 carbon atoms, and benzyl, optionally substituted at any one position of the phenyl group with an alkyl group of 1 to 6 carbon atoms; and (v) compounds of Formula VII ##STR19##
 3. A mirror according to claim 2 wherein, in the solution of reversibly variable transmittance,(A) the solvent is propylene carbonate; (B) the solution includes one or two different cathodic compounds, each at a concentration of 0.1M to 0.1M in the solution at 25° C. at zero-potential equilibrium, and (C) the solution includes one or two different anodic compounds, each at a concentration of 0.01M to 0.1M in the solution at 25° C. at zero-potential equilibrium.
 4. A mirror according to claim 3 wherein, in the solution of reversibly variable transmittance,(A) the cathodic compounds are selected from compounds of Formula X ##STR20## wherein R₁₀₁ is n-heptyl, phenyl or benzyl and X₁₀₁ ⁻ is BF₄ ⁻ or ClO₄ ⁻ ; and the compounds of the formula XI ##STR21## wherein X₁₁₂₋ is BF₄ ⁻ or PF₆ ⁻ ; and (B) the anodic compounds are selected from the compounds of Formula XII ##STR22## wherein R₁₂₁ is methyl or phenyl; and the compounds of Formula XIII ##STR23## wherein R₁₃₁ is methyl or phenyl.
 5. A mirror according to claim 4 wherein, in the solution of reversibly variable transmittance, the cathodic compounds are selected from 1,1'-dibenzyl-4,4'-bipyridinium difluoroborate and tetramethylene bis[4(1benzylpyridine-4'-yl)pyridinium]tetra(hexafluorophosphate), the anodic compounds are selected from 5,10-dihydro-5,10-dimethylphenazine and N,N,N',N'-tetramethyl-1,4-phenylenediamine, the total concentration of the cathodic compounds is 0.01M to 0.05M at 25° C., and the total concentration of the anodic compounds is 0.01M to 0.05M at 25° C.
 6. A mirror according to claim 3 wherein, in the solution of reversibly variable transmittance, the anodic compound is 5,10-dihydro-5,10-dimethylphenazine.
 7. A mirror according to claim 4 wherein, in the solution of reversibly variable transmittance, the anodic compound is 5,10-dihydro-5,10-dimethylphenazine.
 8. A mirror according to claim 5 wherein, in the solution of reversibly variable transmittance, the anodic compound is 5,10-dihydro-5,10-dimethylphenazine.
 9. A mirror according to claim 8 wherein, in the solution of reversibly variable transmittance, the cathodic compound is 1,1'-dibenzyl-4,4'-bipyridinium difluoroborate.
 10. A mirror according to any of claim 3, 4, 5, 6, 7, 8 and 9 wherein the solution of reversibly variable transmittance is thickened with 3% (w/w) to 25% (w/w) or acrylic sheet material thickener.
 11. A mirror according to any of claim 3, 4, 5, 6, 7, 8 and 9 wherein the solution of reversibly variable transmittance includes an inert, current-carrying electrolyte, at a concentration of 0.01M to 1.0M in the solution at 25° C., selected from an alkali metal salt of Formula M₃₁ ⁺ X₃₅ ⁻, a tetraalkylammonium salt of Formula M₃₂ ⁺ X₃₅ ⁻, aluminum chloride, aluminum bromide and any combination of the foregoing salts, wherein M₃₁ ⁺ is potassium, sodium or lithium, M₃₂ ⁺ is tetraalkylammonium wherein the alkyl groups are the same or different and are each selected from alkyl of 1 to 10 carbon atoms, and X₃₅ ⁻ is chloride, bromide, BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, ClO₄ ⁻ and NO₃ ⁻.
 12. A mirror according to claim 10, wherein the solution of reversibly variable transmittance includes an inert, current-carrying electrolyte, at a concentration of 0.01M to 1.0M in the solution at 25° C., selected from an alkali metal salt of Formula M₃₁ ⁺ X₃₅ ⁻, a tetraalkylammonium salt of Formula M₃₂ ⁺ X₃₅ ⁻, aluminum chloride, aluminum bromide and any combination of the foregoing salts, wherein M₃₁ ⁻ is potassium, sodium or lithium, M₃₂ ⁺ is tetraalkylammonium wherein the alkyl groups are the same or different and are each selected from alkyl of 1 to 10 carbon atoms, and X₃₅ ⁻ is chloride, bromide, BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, ClO₄ ⁻ and NO₃ ⁻.
 13. A mirror according to claim 1, 3, 2, 4, 5, 6, 7, 8, and 9 wherein, in the electrochromic device, the solution of reversibly variable transmittance is between and in contact with two planar, parallel, spaced-apart walls, at least one of which is transparent and each of which, on the side in contact with said solution, is coated with an electrode layer of electrically conductive material, provided that, if one of said walls is not transparent, it comprises ar reflecting layer of high reflectance material which reflects, through said solution and the transparent wall, light which reaches said reflecting layer after entering the solution through the transparent wall.
 14. A mirror according to claim 10 wherein, in the electrochromic device, the solution of reversibly variable transmittance is between and in contact with two planar, parallel, spaced-apart walls, at least one of which is transparent and each of which, on the side in contact with said solution, is coated with an electrode layer of electrically conductive material, provided that, if one of said walls is not transparent, it comprises ar reflecting layer of high reflectance material which reflects, through said solution and the transparent wall, light which reaches said reflecting layer after entering the solution through the transparent wall. 